"They will physically guard the Unknowns until conditions are no longer safe," said Maj. John Miller of the 3rd U.S. Infantry. “When the weather makes it unsafe to stand outside, the sentinel will move inside the awards room in the amphitheater where he can still see the tomb, but is out of the elements."

The soldiers are guarding the tomb in two-hour shifts.

The tomb has been guarded around the clock since 1948, when the 3rd Infantry Regiment took over.

While on duty the sentinel crosses a 63-foot rubber-surfaced walkway in exactly 21 steps. He then faces the tomb for 21 seconds, turns again, and pauses an additional 21 seconds before retracing his steps.

The number, 21, symbolizes the highest salute accorded to dignitaries in military and state ceremonies, according to the Army.

Legislation has been proposed to create a Tomb of Remembrance for the unidentified remains of fallen troops from the wars in Iraq and Afghanistan and any future conflicts.

The Tomb of the Unknowns holds the remains of an unidentified U.S. soldier from World War I, with remains of unknown service members from World War II and the Korean War adjacent.

The remains of a Vietnam War casualty were interred at the tomb in 1984 but identified in 1998 through DNA testing. The remains of Air Force Lt. Michael Blassie were then moved to a cemetery in St. Louis near where he grew up.